Searching around seems inconsistent across different brands/models/model years (eg Bolt behavior changed recently). As one pedal driving becomes more prevalent might be something in need of a standard.

For Tesla Model Y:

> If regenerative braking is aggressively slowing Model Y (such as when your foot is completely off the accelerator pedal at highway speeds), the brake lights turn on to alert others that you are slowing down.

Source: Model Y manual: https://www.tesla.com/ownersmanual/modely/is_is/GUID-3DFFB07...

Might be a nice addition for other cars, for example, when you aggressively downshift and decelerate that way.